Magnetron output control



Aug. 23, 1960 Filed Feb W. C. BROWN MAGNETRON OUTPUT CONTROL 2Sheets-Sheet l INVENTOR. Mum/1 c? Bra/w Aug. 23, 1960 w. c. BROWN2,950,416

MAGNETRON OUTPUT CONTROL Filed Feb. 15, 1957 2 Sheets-Sheet 2 INVENTOR.M4 am 47 EEO/WY limited States Fat-ens MAGNETRON OUTPUT CONTROL WilliamC. Brown, Weston, Mass., assignor to the United States of America asrepresented by the Secretary of the Air Force Filed Feb. 15, 1957, Ser.No. 640,556

Claims. (Cl. SIS-39.69)

This invention relates to high frequency oscillators of the magnetrontype, and more particularly to the control of magnetron output. Theinvention is directed to methods and means for reducing the incidence ofunwanted frequencies, or modes of oscillation, in the operation of amagnetron.

In oscillators of the type considered here, known as plural cavitymagnetrons, there is commonly an incidental generation of undesiredfrequencies or modes of oscillation accompanying the generation of thedesired frequency or mode of oscillation as, for example, the pi mode.This incidental generation of other modes causes a lowering of theefficiency, and a somewhat erratic functioning, of the magnetron.

Many procedures have been considered as potential correctives orpalliatives for this condition, among them the procedure of strappingalternate vanes together, or the use of an auxiliary cavity coupled tothe magnetron through a suitable transmission line or other couplingtechnique. Another method has involved employment of transmission linetheory by leading off an undesired frequency or mode of oscillation byway of an extra o'r auxiliary element of selected length to resonate atan undesired adjacent frequency.

Each of these procedures or methods has been successful to a degree, buteach is subject to difficulties and complications in manufacture andadjustment. Strapping, for example, tends to eliminate a great deal oferratic changing of modes, but does not actually eliminate the undesiredmodes of oscillation, per se. Instead, there is a tendency for theseparation of modes to increase. Furthermore at frequencies above 10,000

mc./sec. straps become quite small and mechanically difiicult toincorporate, and their small dimensions result in large copper lossesand lowered efficiency. Again, in the use of a transmission line to leadolf unwanted frequencies through an external auxiliary cavity, and to anexternal auxiliary load, the apparatus becomes bulky, difiicult toadjust and expensive. Similarly, in the use of an extra resonant elementthere is a difficult problem of adjustment to the exact length required.Moreover, such procedure can produce the suppression of only onefrequency or mode of oscillation for each such extra element employed.

It is an object of this invention to reduce the incidence of unwantedfrequencies or modes of oscillation by means incorporated within themagnetron. It is a further object of this invention to provide a methodand means to accomplish the above result Whether the magnetron is tunedor untuned, and whether the cavities are strapped or unstrapped; themethod of the invention involving the establishment of a secondary orshunt circuit linking equipotential regions in adjacent cavities in sucha manner as to tend to maintain the magnetron in operation in the pimode, exclusively, by minimizing energy dissipation through said shuntlink.

Other objects of this invention will be apparent as the tta fj PatentedAug. 23, 1969 Figure l.

in the drawings reference numeral 10 designates the shell of themagnetron, 11 the end caps, 12 and 12 the vanes, 13 the cavities, 14 thecathode, 15 the cathode and filament leads, 16 and 16 loops of lossymaterial connecting adjacent cavities, 17 a slot in one or more vanes toaccommodate the connecting loop, and 18 the output loop for delivery ofthe generated power to theantenna or other utilization element.

Operating current is supplied to the cathode by way of leads 15; andwith magnet poles 19 so placed that the magnetic field is parallel tothe axis of the cathode, the oscillations set up in the magnetron willbe at a frequency which is primarily determined by the dimensions of thecavities 13, but other factors may tend to produce oscillations indifferent modes. In the preferred pi mode, the oscillations will be suchthat the flux densities in adjacent cavities will be equal and thevoltages induced equal and out of phase. In all other modes the fiuxdensities will be unequal and the voltages induced will be unequal.

To reduce the incidence of these undesired modes of oscillation and totend to induce the pie mode of oscillation, the present inventionintroduces a flux-linking energy-shunting element 16 and 16 in the formof a loop of lossy material extending into and fastened to the walls oftwo adjacent cavities, as indicated in Figures 1 and 4. Figures 2 and 3show variants of this loop.

Since in the pi mode of oscillation the flux linkages in adjacentcavities are equal and the voltages induced are equal, and 180 out ofphase, there will be zero or near zero current flow in the lossymaterial, and little loss if any, during operation in this preferredmode.

In any other mode of oscillation the flux linkages will be unequal,hence the voltages will be unequal and there will be current flow in thelossy material with resultant loss. Now inasmuch as the inherenttendency of any magnetron is to operate in that particular mode whichtends to minimize losses due to inter-cavity coupling effects, theresult will be the establishment of a marked preference on the part ofthe magnetron to maintain the pi mode of oscillation in order tominimize coupling between the primary (cathode to anode) circuits, onthe one hand, and the secondary (loop 16) circuit on the other. Thus theincidence of unwanted modes is minimized.

While I have described and shown only one loop of lossy materialconnecting adjacent cavities in a vane type of magnetron it isunderstood that more than one or all of the cavities may be so connectedand that the device may be incorporated in magnetrons with any otherform of cavity. Similarly, other modifications within the spirit andscope of the invention will be apparent to those skilled in the art.Hence the invention is to be understood as limited only as indicated inthe appended claims.

What is claimed is:

1. In a magnetron comprising adjacent resonant cavities adapted todevelop voltages that are of equal magnitude, but opposite polarities,during operation in the preferred mode, means'for damping out unwantedoscillations, comprising a secondary circuit connecting two of saidadjacent cavities, said secondary circuit consisting of a single shuntloop having one half its length disposed within one of said two adjacentcavities, and theother half of its length disposed within the other ofsaid two adjacent cavities, said shunt loop being of a compositiontending toestablish'substantial energy dissipation only 7 when suchunwantedoscillations occur. 7 a

2. In a magnetron assembly, an anode element having a circularperiphery, with cavity-forming digits intruding radially inward fromsaid periphery, and mode-locking means disposed entirely within saidperiphery, said mode- 7 locking meansincluding a shunt loop ofrlossymaterial spanning the digital intrusion between two adjacent cavi- Vties, and having its respective termin als in the arcuate. portions ofsaid circular periphery. that form the respec-" ,tive outer boundariesof said cavities. a

3 In a magnetronassernbly, ananode element having a PfiIipheral wall,with cavity-forming digits intruding radially inward from saidperipheral wall, and modelocking means consisting of a shunt loop oflossy material penetrating the digital intrusion between two adjacent 4V cavities, and having its respective terminals in the arcuate portionsof said circular periphery that form the respective outer boundaries ofsaid cavities.

i 4. In a magnetronassembly, an anode element having 5 a peripheralwall, with cavity-forming digits intruding radially inward from saidperipheral wall, and a modelocking loop of lossy material disposed insymmetrically straddling relationship tothe digital intrusion betweentwo adjacent cavities, said mode-locking loop having one-half '10 itslength positioned within one of said cavities, and the other halfpositioned within the other of said cavities;

5. In a magnetron assembly, an anode element having 7 v a peripheralwall, with cavity-forming digits intruding radially inward from saidperipheral wall,- and a loop of 15 lossy material fastened'at its twoends to said peripheral De Vore Feb, 14; 1950' Heising Apr. 178, 1950

